The present invention concerns a micromechanical component, in particular, a pressure sensor and a corresponding equalization method.
Although applicable to any micromechanical components, the present invention will be explained with reference to example embodiments of a micromechanical pressure sensor.
German Patent Application 197 01 055 A1 describes a semiconductor pressure sensor for measuring an externally applied pressure. FIG. 7 shows a plan view of this conventional pressure sensor. A sectioned drawing of this pressure sensor along section line A-Axe2x80x2 is shown in FIG. 8. The pressure sensor is manufactured on a substrate 2 made of silicon which has a [100] orientation. Located on the underside of a membrane 10 is a depression in the shape of a truncated pyramid. Its configuration is such that at the location of the truncated pyramid, only a small residual thickness of silicon material (membrane 10) remains. The delimiting lines of the truncated pyramid are drawn with dashes in FIG. 7 and run parallel to the [110] and [1I 0] directions, whose orientation is indicated in FIG. 7 with arrows 40 and 41. The portion of silicon substrate 2 that is not thinned is also called support 11.
A measuring resistance 4 that extends in the [110] direction is located on membrane 10 close to the membrane edge. Two electrodes 6, which are made of vacuum-deposited aluminum in the exemplified embodiment selected here, are located on support 11; one electrode is configured in front of and one behind measuring resistance 4, as a respective elongated metallization extending perpendicular to measuring resistance 4. A compensating resistance 5 is located at the left electrode, extending perpendicular to the direction of measuring resistance 4 in the [1I0] direction. Compensating resistance 5 is connected at one end to measuring resistance 4 via a connecting conductor 7, and at its other end to electrode 6 via a second connecting conductor 7. Double arrows 30 and 31 indicate mechanical stresses that are relevant in terms of explaining the manner of operation of this conventional pressure sensor with hysteresis balancing.
FIG. 8 shows a cross section through the pressure sensor of FIG. 7. Substrate 2 has a depression that is trapezoidal in cross section and is delimited by support 11 and membrane 10. Measuring resistance 4 is located in the surface of membrane 10. Measuring resistance 4 is implemented by introducing a local doping zone into the silicon material.
The manner of operation of the conventional pressure sensor with hysteresis balancing is as follows. The pressure sensor is mechanically deformed by a pressure acting externally on the pressure sensor. The thickness of support 11 is typically several hundreds of micrometers, whereas the thickness of membrane 10 is typically several micrometers. Because of the resulting difference in stiffness, the mechanical deformation in support 11 is negligible compared to the mechanical deformation in the membrane. The mechanical stress or deformation resulting from the externally applied pressure is illustrated by an arrow 31, whose length is an indication of the deformation. The mechanical deformation is depicted by way of example at a point, namely at the location of measuring resistance 4.
Also present in the pressure sensor is a first deformation 30 whose cause is a mechanical interference stress which in the present case is based on the differing coefficients of thermal expansion of the aluminum of the electrodes and the silicon of substrate 2. A first mechanical stress or deformation 30 of this kind can be associated with each point in the pressure sensor, but only two points in the pressure sensor will be considered. These two points are assumed to be the location of measuring resistance 4 and the location of compensating resistance 5. In the example embodiment selected here, it is assumed that first deformation 30 is identical everywhere.
Measuring resistance 4 and compensating resistance 5 are dimensioned so that their piezoresistive coefficients are of identical magnitude. The absolute electrical resistance values are also assumed to be identical under identical external conditions. The changes in electrical resistance in measuring resistance 4 and in compensating resistance 5 as a result of first deformation 30 are thus of identical magnitude. Because one resistance is positioned in the direction of the deformation and one perpendicular to the deformation, the two changes in resistance have different signs. The total change in resistance resulting from first deformation 30 in the equivalent resistance for the series circuit made up of measuring resistance 4 and compensating resistance 5 is therefore zero. All that remains, therefore, is the change in the measuring resistance as a result of second deformation 31, to which compensating resistance 5 (located on support 11) is not exposed.
The conventional approach to compensating for hysteresis described above has proven to be disadvantageous in that it exhibits only low efficiency and results in a loss of sensitivity.
In the conventional integrated micromechanical pressure sensor, without compensation a hysteresis of the output signal over temperature therefore generally occurs at the converter element. The hysteresis is generally brought about by a plastic deformation of the aluminum conductor paths of the evaluation circuit, which are located in the surrounding region on support 11. If the sensor element is heated to above xcex94T=60 degrees C., the differing coefficients of thermal expansion of aluminum and the silicon substrate cause mechanical stresses of 100 MPa to occur in the aluminum. Above these stress levels, the aluminum begins to flow. Upon cooling, the same happens in the opposite direction.
FIG. 9 illustrates this hysteresis of the mechanical stress as a function of temperature, and FIG. 10 shows the hysteresis of a further conventional pressure sensor without compensation as a function of membrane edge length mk, for various circuit inner radii SIR. In contrast to the example above, it is assumed here that the circuit conductor paths of the evaluation circuit completely enclose the membrane. The chip size is 4 mm. The global effect predominates in the negative hysteresis region, and the edge effect in the positive hysteresis region.
The hysteretic behavior of the overall aluminum wiring of the evaluation circuit has an integral remote effect on the piezoresistances of the converter element, specifically by way of a xe2x80x9cbimetallic deformationxe2x80x9d (aluminum layer on silicon) of the overall sensor element (global effect), and by way of a local effect when the distance from the circuit edge to the piezoresistance is less than 100 micrometers (edge effect).
Depending on the geometric layout, either the global effect or the edge effect predominates. The influencing variables are:
a) Distance from membrane edge to edge of evaluation circuit;
b) Membrane size;
c) Chip geometry;
d) Glass thickness, cut width of glass saw;
e) Glass size;
f) Solder thickness, adhesive thickness, and mounting substrate.
In sensors soldered on the back side, the hysteresis of the solder partially counteracts the influences caused by the top side of the chip (i.e., aluminum wiring of the circuit). The hysteresis of the solder should therefore be taken into account for absolute hysteresis calculation.
Four measuring resistances are conventionally provided for the micromechanical pressure sensor, and are located (depending on type) in the region in which the edge effect or global effect is dominant. They are connected into a Wheatstone bridge whose output signal consequently also exhibits a temperature hysteresis. This temperature hysteresis overlies the actual sensor signal.
The micromechanical component according to the present invention exhibits a high maximum compensation effect and a small minimum step size, as compared to conventional approaches.
It may be possible to compensate for hysteresis levels exceeding 5% (both positive and negative) at the pressure converter element. The minimum step size may be 0.1% hysteresis.
This may occur without sensitivity loss, or only a minimizable sensitivity loss. Cross-coupling to the measuring resistance may be prevented. Lastly, the compensation function may be easily integrated into the circuit. Equalization by way of mask programming may be provided for. As an alternative, optimum compensation may be established on a precursor article.
The present invention provides, in the surrounding region and/or in the membrane region, at least one patch which may be made of a material such that by way of a deformation of the patch or patches relative to the substrate, an analog interference effect may be generated in such a way that the interference effect acting on the measuring resistance may be compensated for.
According to an example embodiment, a compensating resistance device may be provided which may be configured such that one or more compensating resistances may be additionally connected to the measuring resistance, respective patches being provided in the region of the compensating resistances. A positive or negative hysteresis may be counteracted by additionally connecting one or more compensating resistances.
According to another example embodiment, additional connection of the compensating resistance or resistances may be performed selectably, so that from a defined number of compensating resistances, a specific combination that may be additionally connected to the measuring resistance may be selected. This structure may allow the exact compensation effect to be ascertained by severing conductor paths on the completely configured sensor. The structure may also be designed so that a specific compensation effect may be established by way of a mask change.
xe2x80x9cSelectively additionally connectablexe2x80x9d means that a connection may be interrupted by severing conductor paths, or a connection may be created by short-circuiting conductor paths. This may be effected, e.g., by way of burnout segments (such as, with a laser or voltage pulse) or by thyristor zapping. Individual end-of-line equalization for high-precision requirements may thus be possible.
According to another example embodiment, one or more patches that act directly on the measuring resistance may be provided in the membrane region.
According to another example embodiment, one or more patches, for example, annular patches, which may be located in the surrounding region between the membrane edge and the circuit inner radius of the evaluation circuit, may be provided. A negative hysteresis may be compensated for by way of the global effect that may be controllable in this fashion. Compensating resistances may not be needed in this case. The xe2x80x9ccircuit inner radiusxe2x80x9d means the step between the circuit region and the surrounding region (e.g., A1 step).
According to another example embodiment, the material of the patches may be the conductor path material of the evaluation circuit.
According to another example embodiment, the patches may be located above or alongside an associated compensating resistance.
According to another example embodiment, the substrate material may be silicon and the conductor path material may be aluminum.